A low-temperature cable system is an arrangement for conducting electrical current with low losses, utilizing if possible the phenomenon of superconductivity or the more general principle that the resistance of a conductor is reduced at very low temperatures. The very low temperatures which are most desirable are temperatures below the boiling point of helium and, therefore, liquid helium can be used as a coolant directly surrounding the electrical conductor.
To prevent the incursion of heat into the system or the "loss of cold" therefrom, the cryogenic or low-temperature cable must be shielded against the ambient temperature.
A typical low-temperature cable construction, therefore, comprises a plurality of generally coaxial ducts which may be elastically elongatable, e.g. corrugated, to allow for thermal expansion and contraction, the ducts defining between them a plurality of separate compartments.
The innermost compartment, which is traversed by the electrical conductor means, i.e. one or more electrical conductors in a common insulating sheath or in respective insulating sheaths, this central or innermost compartment being traversed by the lowest temperature coolant, preferably liquid helium.
U.S. Pat. No. 3,917,897 describes the delivery of helium coolant to such an inner compartment for a low-temperature cable. The coolant can be delivered to the low-temperature cable via a special terminal, e.g. as described in U.S. Pat. No. 3,885,636. The use of corrugated ducts to permit expansion and contraction is described in U.S. Pat. No. 3,861,022 and the insulating sheath can be of the type described in U.S. Pat. No. 3,781,455.
Directly outwardly of the inner duct, in which the insulating sheath and electrical conductor means are disposed, there can be an evacuated or vacuum compartment forming a shield against the incursion of heat or the loss of cold by conduction and convection. Radiation losses may be prevented by the filling of this evacuated compartment, or any of the evacuated compartments to be described hereinafter, with layers of superinsulation. A superinsulation, as this term is used in the art, is one which comprises a layer of porous thermal nonconductive material and a layer of radiation-reflective material. A multiplicity of such layers may be laminated together in alternating relationship (see the aforementioned patents).
The evacuated chamber can be surrounded by a coolant-filled chamber using a coolant at a temperature somewhat above the temperature of the liquid helium, e.g. liquefied nitrogen. Outwardly of this coolant-filled chamber there can be provided still another evacuated or vacuum chamber.
Because of the complexity of such low-temperature cables and the number of ducts from which they are constituted, the cables, for electrical current transmission over considerable distances, are formed in lengths or sections which must be interconnected at the installation site. The conventional method of connecting these cables has proved to be insufficiently reliable, lacking the satisfactory strength and incapable of protecting the low-temperature condition at the electrical conductor.
Furthermore, the amount of work required to make the connection and its cost is extensive. Conventional connection techniques are frequently not capable of providing a junction which can adequately withstand the thermal stresses to which the system is subject.